Conventional automobile seats generally include spring members attached to a seat frame, a pad material such as a foamed material or cotton placed thereon, and a skin such as a vinyl leather, woven cloth, or leather covered thereon.
On the other hand, seats other than the automobile seats generally include a pad material placed on the frame and covered with a skin, and some of them also include spring members for enhancing the cushioning characteristics.
However, such seats are mostly thick, heavy and costly.
In view of the above, a variety of lightweight, inexpensive and thin seats have been recently proposed, and the inventors of this application have developed thin seats having a lightweight net of a three-dimensional structure that is provided with desired characteristics (spring characteristics, damping characteristics and the like) sufficient for a cushioning material.
A three-dimensional net (hereinafter referred to as 3-D net) employed in such thin seats is formed of a three-dimensional knit fabric, which has a ground fabric formed into, for example, a knit texture or a honeycomb-shaped (hexagonal) mesh. The 3-D net has a three-layered solid truss structure in which an upper mesh layer and a lower mesh layer are connected to each other by a pile layer having a large number of piles. Each yarn of the upper mesh layer and the lower mesh layer is formed by twisting a number of fine threads, while each of the piles is formed of a single thick string to provide the three-dimensional knit fabric with rigidity.
FIGS. 6A and 6B depict a horizontal cross section of a side portion of a conventional seat back cushion having a 3-D net, and particularly depict a lower portion of the seat back cushion that is brought into contact with a lumbar region of the human body.
As shown in FIGS. 6A and 6B, a side portion of a cushioning material 51 is covered with a trim material 52 such as, for example, a cloth to improve the appearance thereof, and an outer edge of the trim material 52 together with a folded portion 51a of the cushioning material 51 is sewn to an outer edge of a trim material 54 and to one end of a tension member 56 to receive a rearward tension, while an inner edge of the trim material 52 together with a portion of the cushioning material 51 is sewn to one end of a tension member 58 to receive a rearward tension.
In the above-described construction, the tension member 58 is so designed as to have a tendency to loosen and, hence, it is likely that the appearance may be lost. As shown in FIG. 6B depicting a sitting condition, the cushioning material 51 sinks, the tension member 58 receives no tension, and the load of the human body is supported by a tension of the 3-D net.
Accordingly, the tension applied to the trim material 52 is not always constant and there is a good chance that wrinkles or slack may occur in the trim material 52.
The present invention has been developed to overcome the above-described disadvantages.
It is accordingly an objective of the present invention to provide a seat structure having an improved appearance by making the tension applied to a trim material formed thereon substantially constant so that neither wrinkles nor slack may occur in the trim material regardless of whether the seat is loaded or not.